Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Problem 17a

Chapter 10, Problem 17a

Explain why the reaction of an alkyl halide with ammonia gives a low yield of primary amine.

Verified step by step guidance

Step 1: Understand the reaction mechanism. The reaction of an alkyl halide with ammonia involves nucleophilic substitution, where ammonia acts as a nucleophile and replaces the halide group in the alkyl halide.

Step 2: Recognize the issue of over-alkylation. Ammonia can react further with the primary amine product to form secondary, tertiary amines, and even quaternary ammonium salts. This leads to a low yield of the desired primary amine.

Step 3: Consider the competing reactions. The primary amine formed initially is also nucleophilic and can attack additional alkyl halide molecules, leading to the formation of secondary and tertiary amines.

Step 4: Explore alternative methods to improve yield. One such method is the use of alkyl azides, as shown in the image. The alkyl azide avoids over-alkylation and can be reduced to the primary amine using hydrogenation (H₂/Pd-C).

Step 5: Analyze the reaction shown in the image. The alkyl halide reacts with azide ion (N₃⁻) to form an alkyl azide, which is then reduced to the primary amine (CH₃CH₂CH₂NH₂) with the release of nitrogen gas (N₂). This method circumvents the issue of over-alkylation and provides a higher yield of the primary amine.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkyl Halides

Alkyl halides are organic compounds containing a carbon chain bonded to a halogen atom (F, Cl, Br, I). They are key substrates in nucleophilic substitution reactions, where the halogen is replaced by a nucleophile, such as ammonia. The structure and reactivity of alkyl halides influence the outcome of reactions, including the formation of amines.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile attacks an electrophilic carbon atom, displacing a leaving group. In the case of alkyl halides reacting with ammonia, the nucleophilic nitrogen in ammonia attacks the carbon bonded to the halogen, leading to the formation of an amine. However, steric hindrance and competing reactions can lead to low yields of the desired product.

Competing Reactions

In reactions involving alkyl halides and ammonia, competing reactions can significantly affect product yield. For instance, secondary and tertiary amines can form through further reactions of primary amines with alkyl halides, leading to a mixture of products. Additionally, elimination reactions may occur, further reducing the yield of the desired primary amine.

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Related Practice

Textbook Question

What is the product of the reaction of bromoethane with each of the following nucleophiles?

c. (CH₃)₃N

d. CH₃CH₂S^-

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Textbook Question

Rank the following alkyl halides from most reactive to least reactive in an S_N1 reaction:

2-bromo-2-methylpentane, 2-chloro-2-methylpentane, 3-chloropentane, and 2-iodo-2-methylpentane.

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Textbook Question

Which substitution reaction takes place more rapidly?

d. CH₃CH₂Cl + I⁻ or CH₃CH₂Br + I⁻

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Textbook Question

Draw the stereoisomers that are formed from the following S_N1 reactions:

a. 3-bromo-3-methylpentane and methanol

b. 3-chloro-3-methylhexane and methanol

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Textbook Question

What is the product of the reaction of bromoethane with each of the following nucleophiles?

a. CH₃CH₂CH₂O⁻

b. CH₃C≡C⁻

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Textbook Question

Explain why a much better yield of primary amine is obtained from the reaction of an alkyl halide with azide ion (^-N₃), followed by catalytic hydrogenation. (Hint: An alkyl azide is not nucleophilic.)

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